Solution and solid state structures of lanthanides(III) and uranyl(II) complexes of tripodal tris(2-pyridyl)-containing ligand on Ph3P(O) platform

Abstract

Reaction of tris[2-(2′-pyridylmethoxy)phenyl]phosphine oxide (L) with f-element nitrates resulted in 1:1 complexes. The isolated complexes of ligand L with La(III), Eu(III), Tb(III), and U(VI) nitrates, and with La(III) chloride were studied in the solid state and solution by IR, Raman, and NMR spectroscopy, X-ray analysis, and also DFT calculations. Composition and structure of the complexes vary with lanthanide cation radius. According to the data of elemental analysis, vibrational spectroscopy, and X-ray diffraction, the ligand is coordinated in tridentate mode in crystalline La(III) and solid Eu(III) complexes: [Ln(O_PO,N,O_eth-L)(H₂O)(O,O-NO₃)₃], and in monodentate mode in crystalline complex [Tb(O_PO-L)(H₂O)₂(O,O-NO₃)₃]. Nitrogen atoms of pyridine fragments of the ligand not involved in coordination produce intra- and intermolecular H-bonds with coordinated water molecules in the second coordination sphere of Ln(III). According to IR, NMR spectrometry, and DFT calculations, the structure of coordination polyhedron of the main species of Ln(III) complexes in acetonitrile solutions is retained including coordination of one of pyridine fragments in the second coordination sphere: [Ln{O_PO,N,(N*),O_eth-L}(H₂O)(O,O-NO₃)₃] and [Tb{O_PO,(N*)-L}(H₂O)₂(O,O-NO₃)₃], where Ln = La, Eu, and N* is the nitrogen atom of pyridine fragment producing intramolecular H-bond with coordinated water molecule. According IR and Raman spectroscopy, ligand L is coordinated in bidentate mode in solid complex [UO₂(O_PO,N-L)(O,O-NO₃)], and uncoordinated nitrogen atoms remain free. Solution structure of uranyl complex is labile and depends on solvent nature. Equilibria of complex species with variable coordination of nitrate ions, ligand L, and with probable involvement of water take place in CD₃CN and CDCl₃ solutions. Photophysical properties of the prepared Eu(III) and Tb(III) complexes were studied. The preliminary assessment of extraction properties of compound L was made. Stability of studied compounds in acetonitrile solutions was examined, and the structure of one of protolysis product of Eu(III) complex, [Eu(LH)(H₂O)(NO₃)₄], was established by micro-IR and X-ray analysis.